The epithelial cell membrane 252-kDa protein (P252) isolated in our laboratory from Bombyx mori midgut was shown to bind strongly with Cry1Aa, Cry1Ab, and Cry1Ac toxins of Bacillus thuringiensis (15). In the current paper, P252 was shown to bind with chlorophyllide (Chlide) to form red fluorescent protein (RFP) complex, termed Bm252RFP, with absorbance and fluorescence emission peaks at 600 nm and 620 nm, respectively. P252 at a concentration of 1 M is shown to bind with about 50 M Chlide in a positively cooperative reaction to form Bm252RFP under aerobic conditions and in the presence of light at 37°C. Various parameters influencing this reaction have been optimized for efficient in vitro chemical synthesis of Bm252RFP. Circular dichroism spectra revealed that P252 is composed of a ␤-structure (39.8% ؎ 2.2%, based on 5 samples) with negligible contribution of ␣-helix structure. When bound to Chlide, the ␤-structure content in the complex is reduced to 21.6% ؎ 3.1% (n ‫؍‬ 5). Since Chlide had no secondary structure, the observed reduction suggests significant conformational changes of P252 during the formation of Bm252RFP complex. Bm252RFP had antimicrobial activity against Escherichia coli, Serratia marcescens, B. thuringiensis, and Saccharomyces cerevisiae with 50% effective concentrations of 2.82, 2.94, 5.88 M, and 21.6 M, respectively. This is the first report ever to show clear, concrete binding characteristics of the midgut protein to form an RFP having significant antimicrobial activity.The actual insecticidal mechanism of Cry toxins of Bacillus thuringiensis and the receptor protein that leads to pore formation are yet to be fully clarified. However, it has generally been accepted that B. thuringiensis Cry1Aa, Cry1Ab, and Cry1Ac insecticidal proteins bind with putative receptor molecules, aminopeptidase N (APN) (21,33,40), and/or cadherinlike proteins (17,28,39). It is believed that after binding with these receptors, hydrophobic ␣-helices in domain I of Cry1A toxins penetrate into the brush border membrane (BBM) of midgut epithelial cells. Then, by small-pore formation on the BBM, these cells lose their homeostasis, ultimately leading to the insect's death (7,22,45). To understand the complete insecticidal mechanism, we have been interested in elucidating the interaction between epithelial cell membrane proteins and Cry toxins.During the search for the Cry toxin binding proteins, we found 252-kDa proteins in the membrane of epithelial cells from the Bombyx mori midgut (14-16). The protein (P252) was purified from a Triton X-100 soluble fraction of BBM vesicles (BBMVs) of B. mori, and it was purified to homogeneity by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and gel filtration chromatography. It was shown to have a molecular mass of 252 kDa by SDS-PAGE. But the molecular size of about 985 kDa obtained from gel filtration chromatography suggests that the protein could be a homotetramer, and this P252 was shown to bind strongly with Cry1Aa, Cry1Ab, and Cry1Ac toxins of B. thurin...

ABSTRACTThe Cry2Aa3 gene was introduced into asporogenicBacillus thuringiensis, and the synthesized protoxin killedBombyx moriandLymantria disparlarvae. Chymotrypsin hydrolyzed the linkages between 49Tyr/Val50 and 145Lys/Ser146 in the protoxin, and 50- and 58-kDa fragments were generated, respectively. Both peptides killed the larvae of both insects.

P252, a 252-kDa Bombyx mori protein located on the larval midgut membrane, has been shown to bind strongly with Bacillus thuringiensis Cry1A toxins (Hossain et al. Appl Environ Microbiol 70:4604-4612, 2004). P252 was also shown to bind chlorophyllide (Chlide) to form red fluorescence-emitting complex Bm252RFP with significant antimicrobial activity (Pandian et al. Appl Environ Microbiol 74:1324-1331, 2008). In this article, we show that Cry1A toxin bound with Bm252RFP and Bm252RFP-Cry1A macrocomplex, with both antimicrobial and insecticidal activities, was formed. The insecticidal activity of Bm252RFP-Cry1Ab was reduced from an LD₅₀ of 1.62 to 5.05 μg, but Bm252RFP-Cry1Aa and Bm252RFP-Cry1Ac did not show such reduction. On the other hand, the antimicrobial activity of Bm252RFP-Cry1Ab was shown to retain almost the same activity as Bm252RFP, while the other two complexes lost around 30% activity. The intensity of photo absorbance and fluorescence emission of Bm252RFP-Cry1Ab were significantly reduced compared to those of the other two complexes. Circular dichroism showed that the contents of Cry1Ab α-helix was significantly decreased in Bm252RFP-Cry1Ab but not in the other two toxins. These data suggested that the reduction of contents of α-helix in Cry1Ab affected the insecticidal activity of the macrocomplex but did not alter the antimicrobial moiety in the macrocomplex of Bm252RFP-Cry1Ab.

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